Presently, thin films are studied using a number of techniques. In one ultrasonic technique, a radio frequency pulse is applied to a piezoelectric transducer mounted on a substrate between the transducer and the film to be studied. A stress pulse propagates through the substrate toward the film. At the boundary between the substrate and the film, part of the pulse is reflected back to the transducer. The remainder enters the film and is partially reflected at the opposite side to return through the substrate to the transducer. The pulses are converted into electrical signals, amplified electronically, and displayed on an oscilloscope. The time delay between the two pulses indicates the film thickness, if the sound velocity in the film is known, or indicates the sound velocity, if the film thickness is known. Relative amplitudes of the pulses provide information on the attenuation in the film or the quality of the bond between the film and the substrate.
The minimum thickness of films which can be measured using ultrasonics is limited by the pulse length. The duration of the stress pulse is normally 0.1 .mu.sec corresponding to a spatial length of at least 3.times.10.sup.-2 cm for an acoustic velocity of 3.times.10.sup.5 cm/sec. Unless the film is thicker than the length of the acoustic pulse, the pulses returning to the transducer will overlap in time. Even if pulses as short in duration as 0.001 .mu.sec are used, the film thickness must be at least a few microns.
Another technique, acoustic microscopy, projects sound through a rod having a spherical lens at its tip. The tip is immersed in a liquid covering the film. Sound propagates through the liquid, reflects off the surface of the sample, and returns through the rod to the transducer. The amplitude of the signal returning to the transducer is measured while the sample is moved horizontally. The amplitudes are converted to a computer-generated photograph of the sample surface. Sample features below the surface are observed by raising the sample to bring the focal point beneath the surface. The lateral and vertical resolution of the acoustic microscope are approximately equal.
Resolution is greatest for the acoustic microscope when a very short wavelength is passed through the coupling liquid. This requires a liquid with a low sound velocity, such as liquid helium. An acoustic microscope using liquid helium can resolve surface features as small as 500 Angstroms, but only when the sample is cooled to 0.1.degree. K.
Several additional techniques not involving generation and detection of stress pulses are available for measuring film thickness. Ellipsometers direct elliptically polarized light at a film sample and analyze the polarization state of the reflected light to determine film thickness with an accuracy of 3-10 Angstroms. The elliptically polarized light is resolved into two components having separate polarization orientations and a relative phase shift. Changes in polarization state, beam amplitudes and phase of the two polarization components after reflection are observed.
The ellipsometer technique requires films which are reasonably transparent. Typically, at least 10% of the polarized radiation must pass through the film. The thickness of metal sample films thus cannot exceed a few hundred Angstroms.
Another technique uses a small stylus to mechanically measure film thickness. The stylus is moved across the surface of a substrate and, upon reaching the edge of a sample film, measures the difference in height between the substrate and the film. Accuracies of 10-100 Angstroms can be obtained. This method cannot be used if the film lacks a sharp, distinct edge or is too soft in consistency to accurately support the stylus.
Another non-destructive method, based on Rutherford Scattering, measures the energy of backscattered helium ions. The lateral resolution of this method is poor.
Yet another technique uses resistance measurements to determine film thickness. For a material of known resistivity the film thickness is determined by measuring the electrical resistance of the film. For films less than 1000 Angstroms, however, this method is of limited accuracy because the resistivity may be non-uniformly dependent on the film thickness.
In yet another technique, the change in the direction of a reflected beam off a surface is studied when a stress pulse arrives at the surface. In a particular application, stress pulses are generated by an ultrasonic transducer on one side of a film to be studied. A laser beam focused onto the other side detects the stress pulses after they traverse the sample. This method is useful for film thicknesses greater than 10 microns.
A film may also be examined by striking a surface of the film with an intense optical pump beam to disrupt the film's surface. Rather than observe propagation of stress pulses, however, this method observes destructive excitation of the surface. The disruption, such as thermal melting, is observed by illuminating the site of impingement of the pump beam with an optical probe beam and measuring changes in intensity of the probe beam. The probe beam's intensity is altered by such destructive, disruptive effects as boiling of the film's surface, ejection of molten material, and subsequent cooling of the surface. See Downer, M. C.; Fork, R. L.; and Shank, C. V., "Imaging with Femtosecond Optical Pulses", Ultrafast Phenomena IV, Ed. D. H. Auston and K. B. Eisenthal (Spinger-Verlag, N.Y. 1984), pp. 106-110.
Other systems measure thickness, composition or concentration of material by measuring absorption of suitably-chosen wavelengths of radiation. This method is applicable only if the film is on a transparent substrate.